1. Field of the Invention
The present invention relates to increases in measurement distance, a measurement range, spatial resolution and a dynamic range of a distributed fiber optic sensor device. The distributed fiber optic sensor device according to the present invention is used in measurement of, for example, strain distribution and temperature distribution.
Priority is claimed on Japanese Patent Application No. 2008-42570, filed Feb. 25, 2008, the content of which is incorporated herein by reference.
2. Description of the Related Art
Light incident into an optical fiber is scattered at many locations within the fiber due to Rayleigh scattering, Brillouin scattering and Raman scattering. Scattered light characteristics, such as optical power or frequency, vary in accordance with optical fiber characteristics (i.e., loss property), physical quantities (i.e., temperature and strain) imposed on the optical fiber and physical quantities occurring in an environment of the optical fiber. Techniques for localization, such as Optical Time-Domain Reflectometry (OTDR), are for measuring or sensing, for example, physical quantities imposed on the optical fiber along the longitudinal direction of the optical fiber by observing changes in the scattered light at locations in the optical fiber. For example, Huai Hoo Kee, G. P. Lees and T. P. Newson, “1.65 um Raman-based distributed temperature sensor”, Electronics Letters, 1999, Vol. 35, No. 21, pages 1869 to 1871 discloses a distributed fiber optic temperature sensor which takes advantage of the fact that power of Raman scattering light varies depending on temperature. Optical fibers, however, are involved in optical loss (about 0.2 dB/km at the wavelength of 1.55 micrometers in usual single-mode fibers), of which relationship with incident light power and a system dynamic range may limit a measurement range. Especially in Raman scattering, since the power of the anti-Stokes beam caused by the scattering is low, a distributed fiber optic temperature sensor which employs the anti-Stokes beam may have an undesirably limited measurement range.
To increase the incident light power is one way to extend the measurement range. When high-power light of 1 W or greater is incident into the optical fiber, however, a nonlinear phenomenon, e.g., stimulated Raman scattering, occurs within the optical fiber. As a result, as shown in FIG. 7, the incident light is converted into a Stokes beam with the most part thereof being shifted to the longer wavelength side, thereby attenuating the incident light.
Methods of improving the optical fiber itself while satisfying characteristics of usual single-mode fibers have yet to be proposed in order to solve the problem of attenuation in the incident light power due to the stimulated Raman scattering within the distributed fiber optic sensor.
Another method is disclosed in Published Japanese Translation No. 2006-517677 of the PCT International Publication, in which changes caused by the stimulated Raman scattering are utilized in measurement so as to improve a distribution measurement performance. In the disclosed method, once the incident light is converted into a long-wavelength Stokes beam by the stimulated Raman scattering within the fiber, sensing performances thereafter will be conducted using the Stokes beam as the incident light, thereby extending a measurement range. In this method, however, since the wavelength of the incident light is converted along the longitudinal direction of the fiber, the scattered light to be observed also has a varied wavelength. A light-receiving side thus requires more numbers of optical filters. Further, along the longitudinal direction of the fiber, the incident light is gradually converted into the Stokes beam by the stimulated Raman scattering, and therefore, the change in the scattered light cannot be measured in that area. In order to address this problem, the area where the change cannot be measured may be displaced by controlling the incident light power. In this case, however, it is necessary to control the incident light power and thus operations and setups may be complicated.
If the incident light power is increased to extend the measurement range as disclosed in the above-described Huai Hoo Kee, G. P. Lees and T. P. Newson, “1.65 um Raman-based distributed temperature sensor”, Electronics Letters, 1999, Vol. 35, No. 21, pages 1869 to 1871, the measurement range is hardly extended due to the stimulated Raman scattering that undesirably limits the range.
If the Stokes beam caused by the stimulated Raman scattering is employed as disclosed in Published Japanese Translation No. 2006-517677 of the PCT International Publication, it is necessary to control the incident light. This may cause the light receiving section to be complicated and undesirably raise the device cost.
The present invention is made in view of the aforementioned circumstances, and an object thereof is to provide a distributed fiber optic sensor device with extended measurement distance, measurement range and dynamic range while maintaining spatial resolution.